It is commonplace to grow animal cells in cell culture. Mass culture of animal cell lines is fundamental to the manufacture of viral vaccines and many products of biotechnology. Biological products produced by recombinant DNA (rDNA) technology in animal cell cultures include enzymes, synthetic hormones, immunobiologicals (monoclonal antibodies, interleukins, lymphokines), and anticancer agents. Although many simpler proteins can be produced using rDNA in bacterial cultures, more complex proteins that are glycosylated (carbohydrate-modified) currently must be made in animal cells.
Many different cell types may be grown in cell culture, and are of great interest to researchers in many different fields. Such cells include stem cells. Stem cells differ from other kinds of cells in the body. All stem cells—regardless of their source—have three general properties: they are capable of dividing and renewing themselves for long periods; they are unspecialized; and they can give rise to specialized cell types.
The specific factors and conditions that allow stem cells to remain unspecialized are of great interest to scientists. It has taken scientists many years of trial and error to learn to derive and maintain stem cells in the laboratory without them spontaneously differentiating into specific cell types. Therefore, understanding the signals in a mature organism that cause a stem cell population to proliferate and remain unspecialized until the cells are needed. Such information is critical for scientists to be able to grow large numbers of unspecialized stem cells in the laboratory for further experimentation.
Stem cells are unspecialized. One of the fundamental properties of a stem cell is that it does not have any tissue-specific structures that allow it to perform specialized functions. For example, a stem cell cannot work with its neighbors to pump blood through the body (like a heart muscle cell), and it cannot carry oxygen molecules through the bloodstream (like a red blood cell). However, unspecialized stem cells can give rise to specialized cells, including heart muscle cells, blood cells, or nerve cells.
Scientists are attempting to find new ways to control stem cell differentiation in the laboratory, thereby growing cells or tissues that can be used for specific purposes such as cell-based therapies or drug screening.
Adult stem cells typically generate the cell types of the tissue in which they reside. For example, a blood-forming adult stem cell in the bone marrow normally gives rise to the many types of blood cells. It is generally accepted that a blood-forming cell in the bone marrow—which is called a hematopoietic stem cell—cannot give rise to the cells of a very different tissue, such as nerve cells in the brain. Experiments over the last several years have purported to show that stem cells from one tissue may give rise to cell types of a completely different tissue.
There are many ways in which human stem cells can be used in research and the clinic. Studies of human embryonic stem cells will yield information about the complex events that occur during human development. A primary goal of this work is to identify how undifferentiated stem cells become the differentiated cells that form the tissues and organs. Scientists know that turning genes on and off is central to this process. Some of the most serious medical conditions, such as cancer and birth defects, are due to abnormal cell division and differentiation. A more complete understanding of the genetic and molecular controls of these processes may yield information about how such diseases arise and suggest new strategies for therapy. Predictably controlling cell proliferation and differentiation requires additional basic research on the molecular and genetic signals that regulate cell division and specialization. While recent developments with iPS cells suggest some of the specific factors that may be involved, techniques must be devised to introduce these factors safely into the cells and control the processes that are induced by these factors.
Human stem cells could also be used to test new drugs. For example, new medications could be tested for safety on differentiated cells generated from human pluripotent cell lines. Other kinds of cell lines are already used in this way. Cancer cell lines, for example, are used to screen potential anti-tumor drugs. The availability of pluripotent stem cells would allow drug testing in a wider range of cell types. However, to screen drugs effectively, the conditions must be identical when comparing different drugs. Therefore, scientists will have to be able to precisely control the differentiation of stem cells into the specific cell type on which drugs will be tested. Current knowledge of the signals controlling differentiation falls short of being able to mimic these conditions precisely to generate pure populations of differentiated cells for each drug being tested.
Perhaps the most important potential application of human stem cells is the generation of cells and tissues that could be used for cell-based therapies. Today, donated organs and tissues are often used to replace ailing or destroyed tissue, but the need for transplantable tissues and organs far outweighs the available supply. Stem cells, directed to differentiate into specific cell types, offer the possibility of a renewable source of replacement cells and tissues to treat diseases including Alzheimer's diseases, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis, and rheumatoid arthritis.
There is a need in the art for methods to enhance proliferation of stem cells in cell culture, to permit more rapid and efficient study of stem cells and their potential uses.
Ultrasonic stimulation creates “microcavitation” or the creation of minute bubbles in a liquid known as “microcavities.” With each sound wave, these bubbles expand and contract, creating tremendous force and turbulence on a microscopic scale. In some cases, this sound wave is powerful enough to collapse the cavities, which causes even more extreme turbulence, high temperatures, and free radicals in the vicinity of the former cavity. These collapses are powerful enough to dislodge or even destroy cells.
Ultrasonic applications rely on these processes—one common use of ultrasound is as an effective cleaning agent; if the intensity is high enough, collapse cavitation is the dominant factor in the cells' environment. This can strip harmful bacteria off of a surface, and even kill a large number of them. The effectiveness of this technique has been proven by applying ultrasound to one end of a glass tube, using frequencies around 100 kHz and intensities around 40 W/cm2—it was found that approximately 88% of the bacteria were removed from the surface of the tube. Similar experiments have been carried out in a variety of situations, including stripping biofilms off of reverse osmosis membranes. Ultrasound is now actively sold to laboratories as a cleaning aid.
As well as dislodging bacteria, very high intensity ultrasound (>10 W/cm2) has been used to kill suspended bacteria—this relies on collapse cavitation to rend the bacteria's membrane.
Applications also exist for lower-intensity ultrasound; it has long been held that ultrasonic waves can improve the rate of bone growth and indeed, almost 80% of North American physiotherapists possess ultrasonic emitters for the purpose of encouraging speedy recovery. A recent study has indicated that only low-intensity ultrasound is effective in this situation, and low-intensity pulsed ultrasound (LIPUS) devices are currently being marketed for this purpose. One method has been revealed in U.S. Pat. No. 4,530,360 to Duarte (1982).
A method of using low-intensity pulsed ultrasound to aid the healing of flesh wounds is shown in U.S. Pat. Application 2006/0106424 A1 by Bachem (2005). The method utilizes ultrasound to increase the phagocytotic action of the human body's macrophages, and relies on similar theoretical principles to accomplish its aims. However, the method provides no solution for the use of ultrasound outside the confines of a wound.
U.S. Patent Application No. US 2003/0153077 A1 details a method in which low-intensity ultrasound can stimulate the growth of biofilms and other cells—by balancing the beneficial turbulence produced by collapse cavitation with its accompanying negative effects, it was found that low-intensity ultrasound can improve growth rates of cells by up to 50%. The experimenters tested their findings on human and bacterial cells, using frequencies from about 20 kHz to about 1 MHz and intensities encompassing the range from 1 to 5000 mW/cm2. Unfortunately, though increased cell growth is beneficial to the fermentation process, the parameters investigated by this group do not provide the optimal rate of protein expression in fermentation processes.